**Core Concept**
The cornea plays a crucial role in refracting and focusing light onto the retina. Its refractive power is essential for maintaining clear vision, and any abnormalities in corneal shape or structure can lead to vision problems.

**Why the Correct Answer is Right**
The cornea's refractive power is primarily due to its shape and the refractive index of its constituent tissues. The cornea has a curved surface that bends light rays, and its refractive index is approximately 1.376, which is higher than the surrounding aqueous humor (1.336). This combination of shape and refractive index allows the cornea to contribute significantly to the eye's total refractive power. The cornea's refractive power is approximately 43 diopters (D), which is roughly 2/3 of the eye's total refractive power.

**Why Each Wrong Option is Incorrect**
**Option A:** A value of 10 D would be too low to account for the cornea's significant contribution to the eye's total refractive power.
**Option B:** A value of 60 D would be too high, as it would exceed the eye's total refractive power and would not be physiologically possible.
**Option C:** A value of 20 D is close but still underestimates the cornea's refractive power.

**Clinical Pearl / High-Yield Fact**
The cornea's refractive power is essential for maintaining clear vision, and any abnormalities in corneal shape or structure can lead to vision problems such as astigmatism or keratoconus.

**Correct Answer: C. 44 D**